Electrical Safety

Georgia Perimeter College

Objectives

  1. Demonstrate an understanding of the hazards of electricity.

  2. Explain what a short circuit is.
  3. Describe the basics of household electrical wiring and circuits.
  4. Explain the difference between a fuse and a circuit breaker.
  5. Explain how electrical shocks occur.
  6. Describe some electrical safety devices that help protect people from electric shocks.
This section addresses, in whole or in part, the following Georgia GPS standard(s):

  • S8CS2. Students will use standard safety practices for all classroom laboratory and field investigations.

  • S8CS4. Students will use tools and instruments for observing, measuring, and manipulating equipment and materials in scientific activities utilizing safe laboratory procedures.
    c. Learn and use standard safety practices when conducting scientific investigations.
  • S8P5. Students will recognize characteristics of gravity, electricity, and magnetism as major kinds of forces acting in nature.
    b. Demonstrate the advantages and disadvantages of series and parallel circuits and how they transfer energy.

This section addresses, in whole or in part, the following Benchmarks for Science Literacy:
  • Electrical energy can be produced from a variety of energy sources and can be transformed into almost any other form of energy. Moreover, electricity is used to distribute energy quickly and conveniently to distant locations.
  • Make safe electrical connections with various plugs, sockets, and terminals.
  • Troubleshoot common mechanical and electrical systems, checking for possible causes of malfunction, and decide on that basis whether to make a change or get advice from an expert before proceeding.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Perfectly designed solutions do not exist. All technological solutions have trade-offs, such as safety, cost, efficiency, and appearance. Engineers often build in back-up systems to provide safety. Risk is part of living in a highly technological world. Reducing risk often results in new technology.
  • Technological designs have constraints. Some constraints are unavoidable, for example, properties of materials, or effects of weather and friction; other constraints limit choices in the design, for example, environmental protection, human safety, and aesthetics.

  • The potential for accidents and the existence of hazards imposes the need for injury prevention. Safe living involves the development and use of safety precautions and the recognition of risk in personal decisions. Injury prevention has personal and social dimensions.
  • Students should understand the risks associated with natural hazards (fires, floods, tornadoes, hurricanes, earthquakes, and volcanic eruptions), with chemical hazards (pollutants in air, water, soil, and food), with biological hazards (pollen, viruses, bacterial, and parasites), social hazards (occupational safety and transportation), and with personal hazards (smoking, dieting, and drinking).
  • Safety and security are basic needs of humans. Safety involves freedom from danger, risk, or injury. Security involves feelings of confidence and lack of anxiety and fear. Student understandings include following safety rules for home and school, preventing abuse and neglect, avoiding injury, knowing whom to ask for help, and when and how to say no.
  • Some resources are basic materials, such as air, water, and soil; some are produced from basic resources, such as food, fuel, and building materials; and some resources are nonmaterial, such as quiet places, beauty, security, and safety.

Electrical Safety

Electricity plays a significant role in every aspect of our daily lives. From lightning during storms, household electrical circuits to the mechanisms for the control and coordination of our bodily functions (such as heart beat, breathing, and muscle movement) via nerve cells.

Bad or damaged electrical wiring may result in: 

  1. Fatal electric shocks
  2. Electrical fire
  3. Overheating (too hot wires),
  4. Overload (too much current) and
  5. Damage of appliances.

Therefore, the importance of electrical safety cannot be over emphasized.

When humans become part of an electric circuit, currents exceeding 0.10 amps through our bodies can be harmful!

In household circuits operating at 120 VAC (i.e,. AC for alternating current), two heavy-wire power lines from the electric company to the house have voltages of +120 V ("hot" or "live" wire) and -120 V ("neutral"). The third line is the common "ground" (0 volt) wire and is connected directly to the earth.

Connecting between the 'live' line or neutral line (+120 V or -120 V) and the ground (0 V) gives a voltage of 120 V.

Connecting across both (live and neutral) lines gives a voltage of 240 V, which meets the power specifications of some electric stoves and air conditioners.

 

 

Short Circuits

A short circuit is described as a connection that allows current to take an unintended bypass in a circuit.

Worn out insulation on (live) wires may cause the exposed or bare wires to touch each other or touch the ground wire. Such contact between the wires causes the current to take an effectively shorter path instead of following the neutral wire or the ground wire. This is called a short circuit.

Sometimes, a short circuit results when a person accidentally touches a bare live wire and becomes a part of the circuit by creating a short circuit to ground through his or her body. Usually a short circuit provides a lower resistance path than the intended path. The low-resistance path can result in very large current through the short circuit.

Never let your body become a part of an electric circuit!
Do not touch bare live wires or power lines that have fallen.
Do not use worn-out or broken wires.
Always unplug an electrical appliance before attempting to repair it.

Circuit Overloads and Overheating

The household electric circuits are parallel circuits, as shown above. These parallel branch circuits extend out from the "hot" lines to wall outlets, appliances, and lights in rooms. As more appliances are added to a parallel circuit, the combined (equivalent) resistance decreases and the total current increases.

When the wires carry more than the safe current they were designed for, the wires are said to be overloaded. These wires become overheated (joule heat) and can melt the insulation on the wire. These conditions often result in a fire. Fuses and circuit breakers are connected in series with the supply (hot) lines to prevent overloading of circuits.

A fuse is essentially a thin strip of metal with a low melting point. When the current in a fused circuit exceeds the specific fuse rating (e.g. 15 or 20 amp), the joule heat melts or vaporizes the fuse metal strip or "blows". When the fuse blows, it breaks or opens the circuit and cuts off the electric current through those appliances connected to that parallel circuit. The fuse should only be replaced with a new fuse after identifying and correcting the problem that caused the overload.

A circuit breaker uses an electromagnet to shut off the circuit, when the current exceeds a preset current rating, by "tripping" a spring-loaded switch. When the preset current is exceeded, the magnetic field associated with the current is strong enough to pull the spring-loaded switch and disconnect the circuit. The circuit breaker is reset by flipping the switch back to its original position. Circuit breakers are more commonly used instead of fuses to protect utility power supply lines.

Avoid overloading an electrical outlet by plugging too many appliances into it. Never insert your finger or any other objects other than an electric plug into an electrical outlet.

Electric Shocks

A loose connection or bare wire may cause a live wire inside an electrical appliance (e.g. a toaster oven) to "short" by making contact with the metal case or housing of the appliance. Touching the metal case of this appliance may result in a possibly harmful electric shock.

The severity of an electric shock depends on the current, and this electric shock current depends on the voltage and resistance of the human body. The voltage is determined by the source of the electric shock.

Rubbing a balloon with wool will charge the balloon to about 5000 V. But this high voltage does not result in a noticeable electric shock due to the extremely low current (less than 0.0005 amp). The mild electric shock felt from the door knob, after walking and scuffing your feet across a dry carpet, is due to charge flow that results in electric currents of about 0.001 amp.

A 0.100-amp current might cause irregular heart beat and disrupt blood flow through your body. The current passing through a 60-W bulb operating on 120 V is a relatively-high current of 0.500 amp, which is fatal to humans. Due to the ions contained in human cells, living organisms have a low resistance. The resistance of the human body can be as high as 500,000 ohms when the skin is dry, but can be as low as 100 ohms when the skin is wet with salt water. From Ohm's law, as with circuit components, the current drawn by the human body can be defined as:

I = V/ Rbody

So, a low resistance (especially when wet) results in high current through the body! It is the electric current that kills, not the high voltage!

Never handle electrical appliances with when wet or while standing in water!

Other Safety Devices

Other safety devices (other than fuses and circuit breakers) for protecting people from electric shocks and electrical damage of property include three-pronged plugs, polarized plugs, and ground-fault interrupters (GFI).

Three-pronged plugs

Electrical systems are grounded to protect people from electric shocks in case of a short circuit. The three-pronged plug has two flat prongs that connect the appliance to the hot lines of the household circuit. The third prong (grounding wire) is usually rounded and connects the casing of an appliance directly to the ground. Therefore, if a short circuit occurs in the appliance, the current will pass through the low-resistance grounding wire rather than through the person.

Polarized plugs

Of the two flat prongs on the device plug, one of the prongs is larger and wider that the other. The smaller prong plugs into the hot line on the wall socket while the larger prong plugs into the matching slit on the socket connected to the neutral line of the AC circuit. These polarized plugs always has the hot line on the same side of the circuit and the circuit switch is always placed on this hot line  to avert the safety hazard during a short circuit.

Ground-fault interrupters (GFI)

Ground-fault interrupters (GFI) protect circuits by monitoring the current through the hot line and the return current through the neutral line. Normally, the current in the hot line and the neutral (system ground) line should be the same. In a short circuit, some amount of current in the hot line might "leak out" and be diverted directly to ground or to the appliance ground such that the return current in the neutral line is less. The GFI device "trips" almost immediately it detects a difference between the hot line current and the neutral line current, usually within a fraction of a second. The GFI device is usually placed in an outdoor or bathroom circuit where there is danger when wet. The GFI also protects appliances from electric power line surges (large power fluctuations) during electrical storms or accidents with power lines.
 

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Content provided by Mr. Martin O. Okafor, Georgia Perimeter College

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 11, 2007
Modified May 19, 2007